1. Field of the Invention
The present invention relates to a color demodulation device and method for reproducing color difference signals included in a television signal, namely R-Y and B-Y signals. The present invention also relates to an image display device including the color demodulation device so as to produce color image.
2. Description of the Related Art
To produce color image from an analog video signal, for example, the composite video signal, an image display device includes a color demodulation device. The color demodulation device reproduces R-Y and B-Y signals from a chrominance signal included in the composite video signal. The chrominance signal is a sum of color subcarrier signals modulated by the respective R-Y and B-Y signals, and these color subcarrier signals are 90° out of phase each other. The color subcarrier frequency (fsc) is approximately 3.58 MHz. The chrominance signal includes a burst signal consisting of at least eight cycles of the color subcarrier signals, which is 90° out of phase with respect to the color subcarrier signal modulated by the R-Y signal and is 180° out of phase with respect to the color subcarrier signal modulated by the B-Y signal, as shown in FIG. 1. In a conventional color demodulation device, the chrominance signal is sampled at the timing of a burst-lock clock signal, that is a 4 fsc clock signal synchronized with the burst signal. As shown in FIG. 1, the chrominance signal is sampled at each peak of the color subcarrier signals modulated by the respective R-Y and B-Y signals, thereby alternatively producing digital samples of the R-Y and B-Y signals.
The burst signal is not necessarily in phase with the horizontal sync signal HD. Therefore, the digital samples of the respective R-Y and B-Y signals, which are produced at the timing of the burst-lock clock signal, need to be sampled again at the timing of a clock signal synchronized with the horizontal synch signal HD. In other words, the image display device with the conventional color demodulation device requires another clock signal synchronized with the horizontal synch signal HD, as shown in FIG. 2. The sync filter 1 extracts a vertical sync signal VD and the horizontal sync signal HD from the composite video signal. The line-lock clock generator 4 generates a line-lock clock signal synchronized with the horizontal sync signal HD. The line-lock clock signal is supplied to three sets of AD converters 8, a matrix/image processor 11, and a display 10. The display 10 may be a liquid crystal panel (LCD), a cathode ray tube, or a plasma display panel (PDP).
The burst signal filter 3 extracts the burst signal from the composite color video signal. The burst-lock clock generator 22 generates the burst-lock clock signal based on the burst signal extracted by the burst signal filter 3. The burst-lock clock signal is supplied to an AD converter 20, a YC filter 21, a color modulator 23, and three sets of DA converters.
The AD converter 20 samples the composite video signal using the burst-lock clock signal as a sampling clock. The YC filter 21 separates digital samples of a luminance signal (Y) and a chrominance signal (C) included in the digital samples of the composite video signal. The color demodulator 23 produces the digital sample of the respective R-Y and B-Y signals from the digital samples of the chrominance signal. The digital samples of the respective R-Y, B-Y and Y signals are converted to analog signals by the DA converters 24. The AD converters 8 sample the analog signals produced by the DA converters 24 at the timing of the line-lock clock signal, thereby producing the digital signals of the R-Y and B-Y and Y signals synchronized with the line-lock clock signal. The matrix/image processor 11 converts the digital samples of the R-Y, B-Y and B-Y signals into that of R, G, and B signals. Such a structure shown in FIG. 2, using two clock signals, i.e. the burst-lock clock and line-lock clock signals, appears in Japanese Patent Laid-Open No. 298913/1999.
As discussed above, in the conventional color demodulation device, the chrominance signal has to be sampled at the timing of the burst-lock clock signal, which is precisely synchronized with the burst signal. In other words, if a phase of the burst-lock clock signal deviates from that of the burst signal, the digital samples of the R-Y and B-Y signal will not be produced accurately. Furthermore, the image display device with the conventional color demodulation method requires two different clock signal generators, which not only increase circuit size but also cause noise resulting from interference of different clocks.